Optical wavelength converter

ABSTRACT

An optical wavelength conversion device including a micro-structured optical waveguide, which includes sections with a non-linear material having an index of refraction which changes as a non-linear function of light intensity. The optical waveguide includes a light guiding core region, and is dimensioned for providing spatial overlap between the sections filled with the non-linear material and light propagating within the waveguide. First and second optical light sources may also be included, the second light source having an intensity sufficient to change the refractive index of the non-linear material sufficiently to encode or modulate the light from the first light source through the effect of leaking light from the first light source inside the guiding core to the outside of the guiding core.

FIELD OF INVENTION

The present invention relates to optical devices for conversion ofoptical signals from one wavelength to another, and in particular suchdevices realised using micro-structured optical waveguides.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing (WDM) is considered one of the mostfeasible ways to upgrade point-to-point transmission links and to meetthe huge demand for transmission capacity in telecommunication systems.As the network technology evolves, it will become important forreconfiguration purposes to be able to perform wavelength conversion ortranslation within the network or at its interfaces. In order to meetthe transmission capacity demands of future optical systems it will beimportant to make all-optical wavelength converters, and recent researchhave focussed on various means of providing such functionality (for areview see Stubkjaer et al. IEICE Trans. Electron. Vol. E82-C, No. 2,February 1999). The focus in the research has been on wavelengthconversion on binary signal formats that are by far dominant fortelecommunication traffic today. As described by Stubkjaer et al., theconverters fall into four groups: The first is opto-electronicconverters, which consists of a detector followed by amplification orregeneration and transmitter stages. For present and future high-speedcommunication links (operating at bit rates at 10 Gbit/s or more), thepower consumption of the opto-electronic converter will be high andbandwidth limitations of electronic circuitry are likely to beencountered. A second group of converters includes the laser convertersthat are working by optical control of single frequency lasers throughthe effect of an input signal launched into the laser causing gainsaturation, which in term controls the oscillation of the laser. Theintensity modulated (IM) output is associated with chirp, which willlimit transmission on non-dispersion shifted optical fibres, and themaximum bit-rate (determined by the laser's resonance frequency) islimited to around 10 Gbit/s. The third group of optical converters—theso-called coherent converters—rely on four wave mixing (FWM), and theymay be realised using either optical fibres or semiconductor opticalamplifiers (SOAs) as non-linear elements. Coherent converters aretransparent to signal format, so many WDM channels may be convertedsimultaneously, but their conversion efficiency is normally low. A moreserious drawback of the FWM converter is the dependency of the outputwavelength on both the pump and the input signal wavelengths, and thattwo pumps will be needed to ensure polarisation insensitive operation.The fourth group of optical wavelength converters described todayincludes converters based on optically controlled gates. According toStubkjaer et al., these converters seem to be the most promisingall-optical converters, working by the principle of letting the inputpower control the gating of CW light at either a fixed or tuneablewavelength. Numerous approaches have been studied concerning theimplementation of the optical gates, including the use of semiconductoroptical amplifiers (SOAs)—through gain saturation or in interferometricwaveguide configurations—or multi-section SOAs or optical fibres innon-linear optical loop mirror (NOLM) configurations. Yet another typeof gate has been achieved for cross-absorption modulation inelectro-absorption modulators. As concluded by Stubkjaer et al., recentresearch results on all-optical converters have shown excellentproperties—particularly for converters based on optically controlledgates—and research in packaging and “second generation” devices is underway.

It is, however, a disadvantage of the present all-optical convertersthat they include the use of relatively complex semiconductor opticalamplifiers or (as a less studied alternative) long fibre devices forobtaining the desired optical gating properties. It is, therefore,highly relevant to look for alternative physical mechanisms to providethe optical gating, which may eventually lead to improved opticalwavelength conversion.

An interesting possibility concerning the development of new opticallycontrolled switches is described in the patent application U.S. Pat. No.5,907,647 entitled “Long-period grating switches and devices using them”by Eggleton et al. This invention describes means to obtain an opticalswitch by employing a long-period fibre grating for switching lightbetween alternative optical paths. The fundamental elements of thedevice comprise a variable intensity light source, a length of opticalwaveguide dimensioned for co-propagating light in two distinguishablemodes, and a long-period grating in the waveguide for coupling betweenthe two modes. The waveguide is non-linear so that the effectiverefractive index is a function of intensity, and as a consequence, thecoupling produced by the grating is a function of intensity, anddifferent levels of light intensity can switch between the separatemodes of the fibre waveguide. In the description by Eggleton et al.,examples of the non-linear function of the waveguide such as glass(possibly doped with telluride or selenide) at sufficiently highintensities or other materials such as semiconductors or organic layeredmaterials are mentioned

An advantage of the invention described in U.S. Pat. No. 5,907,647 isthat the signal source and the control source can have the same ordifferent wavelengths, and they can pass through the waveguide in thesame or different directions. It is, however, a disadvantage that thedevices described by the invention of Eggleton et al. require theinclusion of a long-period waveguide grating. More specifically, it isdescribed by Eggleton et al., how coupling between modes of thewaveguide will not take place in the absence of the long-period grating,and, consequently, the switch described by Eggleton will not work, ifthe grating is omitted.

In addition to this, it is not described in U.S. Pat. No. 5,907,647, howthe long-period grating switches may be used in a wavelength conversionset-up, and it will add complexity to the system to perform thenecessary filtering of higher order modes (by the mode separators)discussed in the description by Eggleton. There is, consequently, a needfor alternative waveguide components in order to obtain the desiredwavelength conversion in optical WDM networks.

Recently, James S. Shirk and Armand Rosenberg (Laser Focus world, April2000, pp. 121–129) published results on optical limiters usingnon-linear photonic crystals fabricated by incorporating organic dyeshaving a non-linear absorption or refraction. The overall idea behindthese components is that the channels of a micro-structured (ornanochannel) glass structure are filled with a non-linear material,whose low-intensity refractive index matches that of glass. When intenselight is incident on the structure, the index of the non-linear materialis altered, resulting in the development of a bandgap, and that thetransmission through the device will eventually drop. As described byShirk and Rosenberg, recent studies of the optical properties ofindex-matched phthalocyanine (Pc) dyes have led to the development ofmaterials that combine a large non-linear absorption with largenon-linear refraction. Some of these materials have been designed to beliquid or low-melting-point glasses, so that they can be used to fillthe tiny open channels in a nanochannel glass crystal. For illuminationperpendicular to the nanochannels, as reported in the article by Shirkand Rosenberg, improved limiting properties are observed at short timesand for low fluences in the photonic crystal. The reason is thatcontributions from the non-linear absorption and the photonic bandgapformation combine and add their effects. This is an improvement over thepreviously described optical limiters using a photonic crystal structurecontaining a thermal non-linear liquid, such as the one described by Linet al., Optics Letters, Vol. 23, No. 2, January 1998.

An additional advantage of the limiter described by Shirk and Rosenbergis that because the decrease in transmission is due, in part, to anincrease in reflectivity, the energy load on the non-linear material isreduced, lowering the potential for damage to the material.

Limiting is observed for 5 ns laser pulses, and the transmitted energyis limited to just over 250 nJ, i.e., the functionality of the device isdemonstrated even at modest power density levels.

The work of Shirk and Rosenberg does not involve optical waveguides, andneither does it describe optical switching using non-linear propertiesof photonic bandgap structures. Some aspects of optical switching may,however, be found in the paper by Scholz, Hess and Rühle, entitled“Dynamic cross-waveguide optical switching with a non-linear photonicband-gap structure”, published in OPTICS EXPRESS, Vol. 3, No. 1, July1998. In this paper, a numerical study of a two dimensional all-opticalswitching device, which consists of two crossed waveguides and anon-linear photonic band-gap structure in the centre. The switchingmechanism is based on a dynamic shift of the photonic band edge by meansof a strong pump pulse, and it is modelled on the basis of a twodimensional finite volume time domain method. The described methodsolely considers orthogonal propagation of pump beam and signal beam,and the mentioned waveguiding is limited to two dimensions, i.e., thelight is not confined in the third dimension. The work is, however,interesting because it indicates switching times in the order of 10⁻¹⁴seconds.

It is a disadvantage of the switching device described by Scholz, Hess,and Rühle that waveguiding in three dimensions is not described, andthat light confinement through optical waveguiding is not utilized.

It is a further disadvantage of the switching device described byScholz, Hess and Rühle that more than one wavelength is not involved inthe switching process.

It is still a further disadvantage of the switching device described byScholz, Hess and Rühle that the orthogonally propagating pump light andsignal light, only has a short interaction length, and thereforerequires a very high non-linear response of the material. This will putlimits to the wavelength range within which the photonic bandgap edgesmay be moved in practice.

It is still a further disadvantage of the switching device described byScholz, Hess and Rühle that the device works by switching betweenguidance or reflection of the signal light, and therefore results in asignificant reflected signal power.

It is well known that by the use of optical waveguides, the localintensity of optical mode fields may be locally enhanced, as it is wellknown from non-linear optical fibres.

It is a disadvantage of the optical limiter described by Shirk andRosenberg that the optical limiters are not applying the property ofoptical waveguiding.

It is a further disadvantage of the optical limiters described by Shirkand Rosenberg, that they are not making use of the full wavelength spanof the non-linear materials, since they fundamentally look at theability to modify the reflection and transmission properties of a singlewavelength through the PBG material.

It is a further disadvantage of the invention described by Eggleton thatit does not make use of the improved waveguiding properties ofmicro-structured optical waveguides, and especially the new propertiesof photonic bandgap waveguides.

It is the object of the present invention to provide a new class ofoptical waveguides, in which the transversal waveguiding structure willbe strongly dependent on the optical power level of the modesilluminating or propagating within the structure.

It is a further object of the present invention to provide anall-optical switching element in which the new class of non-linearoptical waveguide is a key element.

It is still a further object of the present invention to provide anall-optical wavelength converter with potential applications especiallywithin the area of telecommunication technology.

SUMMARY OF THE INVENTION

The present inventors have realised that the non-linear properties ofoptical limiters using non-linear photonic crystals may be enhanced incombination with a three dimensional optical waveguiding structure.

Furthermore, the present inventors have realised how to design a rangeof optical waveguides enhancing the non-linear properties needed for thewavelength conversion according to the fundamental idea. The waveguidesare based on either the total internal reflection principle or thephotonic bandgap principle. The specific design parameters of preferredembodiments are described in the following part of this description.

The fundamental problem to be solved by the invention is how to transferoptically encoded signal information from one wavelength to anotherwavelength in an efficient manner using a non-linear microstructuredoptical waveguide, which waveguiding properties are significantlyaltered as a function of the optical intensity of the encoding light.

In a first aspect of the invention there is provided an opticalwavelength conversion device (which e.g., can be used in an opticalfibre communication system) that comprises a micro-structured opticalwaveguide including sections comprising a non-linear material having anindex of refraction, which changes as a non-linear function of lightintensity. The optical waveguide comprises a light guiding core regionin or around which an optical signal may be guided, and the waveguide isdimensioned to provide spatial overlap between the non-linear materialand the light propagating within the waveguide. In the preferredembodiment, the wavelength conversion device further comprises a firstoptical light source for introducing light into said waveguide in amode, which may be guided along the core at low power intensities. Inorder to modify the waveguiding properties for this first optical lightwavelength, it is also preferred that the conversion device comprises asecond intensity-modulated light source for introducing light into saidwaveguide in such a manner that it illuminates the sections filled withthe non-linear material, said second light source having an intensitymodulation sufficient to change the refractive index of the non-linearmaterial by an amount sufficient to encode or induce modulation of thelight from the first optical light source. The encoding or modulationthereby takes place through the effect of leaking light from the firstlight source from the inside of the guiding core to the outside of theguiding core.

It is preferred that the core region of the micro-structured opticalwaveguide is surrounded by a cladding region.

Since the present invention relates to the application of opticalwaveguides in which the light intensity levels have to be wellcontrolled, it is highly useful to combine the wavelength converter withelements or spatially separate optical elements comprising amplifyingmeans for adjusting the optical power levels to obtain improvedperformance. Here, the optical power levels of the first and/or secondlight source may be adjusted.

A very high degree of flexibility in waveguide design combined withamplification efficiency is to design a wavelength converter in whichthe amplifying element is a rare-earth doped waveguide section. For thepreferred telecommunication applications today, rare-earth materialssuch as erbium and/or ytterbium may be a preferred choice, but as theoptical wavelength band is expanded in future systems, other rare-earthmaterials may be of equally high relevance.

It is a very interesting aspect of the present invention that it doesnot limit itself to the fundamental type of waveguiding principle (i.e.,it is neither limited to total internal reflection nor to the photonicbandgap effect). One possibility is, therefore, to design opticalwaveguides containing micro-structured features filled with opticallynon-linear material, wherein the waveguide core is realised with araised refractive index compared to the refractive index surrounding thecore, i.e., the waveguide is operating by the principle of totalinternal reflection.

As it will become clear from the following discussion of examplesdescribing preferred embodiments, it will be advantageous to realise awavelength converter according to the invention, and guiding light bythe physical principle of total internal reflection in such a mannerthat the wavelength of the encoding light wavelength λ₁ relates to thewavelength of the encoded light λ₂, according to the relation λ₁<λ₂,since this will allow for the encoding wavelength to maintain guided,while the longer wavelength will be allowed to leak out of thewaveguiding structure.

It is also within an embodiment of the first aspect of the inventionthat the wavelength converter may be realised by designing a waveguidecontaining features filled with non-linear material and furthercontaining more than one core element or core region. This will allowthe optical power to be coupled between the different core regions,further allowing spatial separation of the output from the wavelengthconverter. The number of core elements may for example be 2 coreelements, 3 core elements, 5 core elements, or 10 core elements.

As already mentioned, the invention also works in cases where thewaveguiding may be controlled by the photonic band-gap effect, i.e., incases in which the waveguide core region is realised with a loweredrefractive index compared to the refractive index surrounding the core,and in which the micro-structured waveguide comprises a multiplicity ofspaced apart cladding-structure features that are elongated in thewaveguide axial direction and disposed in the waveguide material or thecladding region. In this case the cladding structure may have a periodicdistribution of the micro-structured features.

The first aspect of the invention further covers embodiments in whichthe wavelength converter may be realised with a low-index core—guidingby the PBG effect—and for such designs, very feasible dimensions may beobtained, if the wavelength of the encoding light wavelength λ₁ relatesto the wavelength of the encoded light λ₂, according to the relationλ₁>λ₂. Hereby, a very sensitive movement of the guided mode may beobtained within the photonic band-gap, while working with dimensions ofthe periodicity of the cladding structure that can be manufactured withtodays technology.

The wavelength converter according to the first aspect of the inventionmay also be realised having more than one PBG-guiding core elementsbetween which the optical power of one optical wavelength, λ₂, may becoupled in a manner controlled by the optical power of a second opticalwavelength, λ₁. The number of core elements may for example be 2 coreelements, 3 core elements, 5 core elements, or 10 core elements.

It is preferred that the wavelength converter according to the firstaspect of the invention may be realised with the micro-structuredoptical waveguide comprising an optical fibre. Several advantages areobtained by this realisation, and among the central are a very easyadaptation to existing optical communication systems, and access to themost mature waveguide technology within the area of photonic band-gapwaveguides.

However, as an alternative, it is also within an embodiment of the firstaspect of the invention that the wavelength converter can be realisedwith the micro-structured optical waveguide comprising an optical planarwaveguide. This has the advantage of allowing integration with otheroptical functionalities on the same waveguide wafer.

One of the advantages of this invention is that the interaction lengthof the light with the waveguiding structure may be made considerable,allowing for the use of a weaker non-linear process or alternativelymaking the devices operate at lower threshold powers. In order for thisto be used most efficiency, a wavelength converter according to theinvention should be made in a manner wherein the encoding lightpredominantly is guided along the same waveguide axis, as the light tobe encoded.

The waveguide realisation of the wavelength-converting device accordingto the invention allows a high degree of interaction control betweenpropagating modes and specific waveguide features. In a furtherpreferred embodiment, of such a wavelength converter the encoding lightand the light to be encoded may, therefore, be co-propagating, allowinga very strong interaction and lower power threshold values.

Yet for other applications, it will be advantageous to physicallyseparate the input port for the encoding light and the light to beencoded. Also in this case the waveguiding property may be of centralimportance, and in a preferred embodiment, a wavelength converteraccording to the invention may have the encoding light and the light tobe encoded counter-propagating.

There will, however, also be applications, where small physicaldimensions, or short interaction length requirements points towardsrealisations, where only one of the light fields are guided. For suchapplications, a preferred embodiment will be a wavelength converteraccording to the invention, wherein the encoding light has a predominantpropagation direction, which is different from that defined by thewaveguide axis.

For practical realisation of the wavelength converter according to theinvention, it is preferred that the core has a diameter larger than 2microns in the cases, where a raised-index core is used, because thesedimensions generally will result in spotsize values that are appropriatefor coupling to components used in standard optical communicationsystems.

As it will be further argued in the discussion of the examples, awaveguide according to the invention, and having a raised-index core,could in a preferred embodiment further be containing a waveguidefeature to which the light may be leaking at high intensities, andhaving the boundary of this feature placed at a distance from the corecentre, which is larger than 0.75 times the core diameter, such aslarger than 1.0 times the core diameter, such as larger than 1.5 timesthe core diameter, such as larger than 2.0 times the core diameter.These specific parameters may depend on the filling fraction ofnon-linear material, the non-linear coefficient, and the specificwaveguide design.

An important design parameter for micro-structured optical waveguides isthe so-called pitch, or centre-to-centre spacing between the nearestcladding-structure features. It should also be noted that these claddingstructure features very often comprise air-filled voids or air holes.Depending on the specific application of the wavelength-convertingdevice and the involved wavelengths, a wavelength converter according tothe invention would in preferred embodiments be designed to have thecentre-to-centre spacing of nearest air holes smaller than 5.0 μm, suchas around 4.0 μm, such as around 3.0 μm, such as around 2.0 μm, such asaround 1.0 μm, such as around 0.5 μm, or even smaller.

As further described in the following examples, a waveguide according tothe invention, and having a low-index core region, may be designed withgreat flexibility by using optimal trade-off between the fillingfraction of the air holes in the cladding and that of the non-linearmaterial. Depending on the specific application, this leads to preferredembodiments in which the features filled with non-linear material has across-section area which is at least 10% as large as the cross sectionarea of the cladding-structure features, such as a cross-section areawhich is at least 20% of the cross-section area of thecladding-structure features, such as a cross-section area which is atleast 40% of the cross-section area of the cladding-structure features,such as a cross-section area which is at least 75% of the cross-sectionarea of the cladding-structure features, such as a cross-section areawhich is at least 100% of the cross-section area of thecladding-structure features, such as a cross-section area which is atleast 200% of the cross-section area of the cladding-structure features.

The present invention also covers embodiments, in which themicro-structured optical waveguide is dimensioned such that the sectionscomprising a non-linear material are placed within a distance from thecore centre which is smaller than 10 times the operating wavelengths ofthe encoding and/or encoded light, such as smaller than 8 times theoperating wavelengths of the encoding and/or encoded light, such assmaller than 6 times the operating wavelengths of the encoding and/orencoded light, such as smaller than 4 times the operating wavelengths ofthe encoding and/or encoded light, such as smaller than 2 times theoperating wavelengths of the encoding and/or encoded light, or such assmaller than 1 time the operating wavelengths of the encoding and/orencoded light.

From the above discussion it should be clear that the first aspect ofthe invention covers an optical wavelength conversion device including amicro-structured optical waveguide. However, the present invention alsocovers a micro-structured optical waveguide itself.

Thus, according to a second aspect of the present invention there isprovided a micro-structured optical waveguide having an axial directionand a cross section perpendicular to said axial direction, themicro-structured optical waveguide including sections that are elongatedin the axial direction and comprising a non-linear material having anindex of refraction, which changes as a non-linear function of lightintensity, said waveguide including a light guiding core region beingelongated in the waveguide axial direction, and said waveguide beingdimensioned for providing spatial overlap between the sections filledwith the non-linear material and the light propagating within thewaveguide. Thus. The transversal waveguiding structure may depend on theoptical power level of the modes illuminating or propagating within thestructure. It is preferred that the core region is surrounded by acladding region. It is also preferred that the micro-structured opticalwaveguide is an optical fibre or comprises an optical fibre.

According to an embodiment of the second aspect of the invention, thewaveguide core may be realised with a raised refractive index comparedto the refractive index surrounding the core.

However, it is also within an embodiment of the second aspect of theinvention that the waveguide core region may be realised with a loweredrefractive index compared to the refractive index surrounding the core.Here, the waveguide may comprise a multiplicity of spaced apartcladding-structure features that are elongated in the waveguide axialdirection and disposed in the cladding region surrounding the coreregion.

According to embodiments of the second aspect of the invention, thewaveguide may comprise a plurality of core regions or elements, such asfor example 2 core elements, 3 core elements, 5 core elements, or 10core elements.

In a preferred embodiment of the second aspect of the invention, thecore region may have a diameter larger than 2 microns. It is alsopreferred that the waveguide further comprises a cladding zone to whichthe light may be leaking at high intensities, and having an innerboundary of this zone placed at a distance from the core centre, whichis larger than 0.75 times the core diameter, such as larger than 1.0times the core diameter, such as larger than 1.5 times the corediameter, or such as larger than 2.0 times the core diameter.

When the waveguide according to the second aspect of the inventioncomprises cladding structure features in the cladding region, thesewaveguide cladding structure features may be air holes, and thecentre-to-centre spacing of nearest air holes may be smaller than 5 μm,such as around 4.0 μm, such as around 3.0 μm, such as around 2.0 μm,such as around 1.0 μm, such as around 0.5 μm, or even smaller.

When the waveguide comprises cladding structure features in the claddingregion, it is also within an embodiment of the present invention thatthe waveguide sections comprising the non-linear material have across-section area, which is at least 10% as large as the cross-sectionarea of the cladding-structure features, such as a cross-section areawhich is at least 20% of the cross-section area of thecladding-structure features, such as a cross-section area which is atleast 40% of the cross-section area of the cladding-structure features,such as a cross-section area which is at least 75% of the cross-sectionarea of the cladding-structure features, such as a cross-section areawhich is at least 100% of the cross-section area of thecladding-structure features, or such as a cross-section area which is atleast 200% of the cross-section area of the cladding-structure features.

According to an embodiment of the second aspect of the invention, it ispreferred that the micro-structured optical waveguide is dimensionedsuch that the sections comprising the non-linear material are placedwithin a distance from the core centre which is smaller than 10 timesthe operating wavelength of the light to be guided by said core region,such as smaller than 8 times the operating wavelength of the light to beguided by said core region, such as smaller than 6 times the operatingwavelength of the light to be guided by said core region, such assmaller than 4 times the operating wavelength of the light to be guidedby said core region, such as smaller than 2 times the operatingwavelength of the light to be guided by said core region, or such assmaller than 1 time the operating wavelength of the light to be guidedby said core region.

The second aspect of the invention also covers embodiments wherein thewaveguide comprises waveguide features containing a photo-sensitivematerial in which permanent refractive-index changes may be induced.

It has already been discussed that the optical waveguide according toembodiments of the second aspect of the present invention may comprise amultiplicity of spaced apart cladding-structure features, which claddingstructure features may be air holes or voids. Here, it is preferred thatthese cladding features are elongated in the waveguide axial directionand disposed in the cladding region, and it is furthermore preferredthat the cladding-structure features predominantly are periodicalcladding-structure features. The cladding-structure features may beplaced in different arrangements, such as placed in a close-packedarrangement, such as placed in a honey-comb structure, or such as placedin a Kagomé structure. When placed in a honey-comb structure, thesections comprising the non-linear material may be placed within adistance from the core centre of one time the distance between thenearest cladding-structure features in the cladding.

In the cases, where the optical waveguides are operating by the photonicbandgap effect, it may be a requirement, that the micro-structuredwaveguide comprises a multiplicity of spaced apart cladding-structurefeatures, which predominantly are periodically distributed within atleast part of the cladding. In cases where total internal reflection isthe fundamental waveguiding principle, it may, however, also be anadvantage to provide a cladding with periodical distribution of thecladding-structure features, e.g., for fabrication simplicity or forobtaining specific waveguiding properties. The use of a close-packedarrangement may simplify the fabrication process for certain waveguidedesigns. It should also be mentioned that a good method for obtainingrelatively large band gaps in PBG guiding waveguides may for example beto place the cladding-structure features in the honey-comb structure orthe Kagomé structure or another non-close-packed structure.

It should be understood that according to embodiments of the secondaspect of the invention, the sections comprising the non-linear materialmay be voids, channels or holes being elongated in the axial directionof the waveguide and arranged in the cladding layer surrounding the coreregion or regions. The sections comprising the non-linear material maybe placed in different arrangements, such as being periodically arrangedin the cladding surrounding the core region(s), such as placed in aclose-packed arrangement, or such as placed in a in a honey-combstructure.

It should be understood that it is within the scope of the presentinvention that the wavelength conversion device according to the firstaspect of the invention also covers embodiments in which themicro-structured optical waveguide is selected from any of theembodiments of the optical waveguide according to the second aspect ofthe invention.

The present invention also covers a third aspect in which there isprovided an optical system comprising a first source of encodingmodulated light, a first length of optical waveguide for carrying saidencoding modulated light, a second source of light to be encoded havinga wavelength different from the wavelength of the first source, a secondlength of optical waveguide for carrying said light to be encoded, awaveguide coupler to join the two light wavelengths into the samewaveguide, a wavelength converting waveguide containing sections filledwith non-linear material and for controllably converting the signalinformation from the first wavelength to the second wavelength, and athird and a fourth length waveguide to guide the light into and out fromthe wavelength converter. Here, the wavelength converting waveguide maybe or comprise a micro-structured optical waveguide selected from any ofthe embodiments of the optical waveguide according to the second aspectof the invention. However, the wavelength converting waveguide mayalternatively be or comprise an optical planar waveguide.

A micro-structured optical waveguide including sections of the waveguidefilled with a material having an index of refraction, which changes as anon-linear function of light intensity and dimensioned to providespatial overlap with the light propagating within the waveguide, may notonly be used to modify the light propagation at modulation frequenciesequal to the signal bit rate. Another possibility is to make use of thenon-linear property, and operate the component by a control wavelength,which is intensity modulated at a much lower frequency rate. This may beused to form an optical switching device comprising a core region in oraround which an optical signal may be guided for low intensities ofoptical power, but as the control light power is increased the bitstream from a first optical light source may be switched off.

Thus, according to a fourth aspect of the invention there is provided anoptical switching device comprising a micro-structured optical waveguideincluding sections comprising a material having an index of refractionwhich changes as a non-linear function of light intensity, said waveguide including a light guiding core region, and said waveguide beingdimensioned for providing spatial overlap between the sections filledwith a non-linear material and light propagating within the waveguide.The optical switching device further comprises a first optical lightsource for introducing light into said waveguide in a mode guided alongthe core, and a second variable intensity light source for introducinglight into said waveguide in such a manner that it illuminates thesections filled with a non-linear material, said second light sourcehaving an intensity variation sufficient to change the refractive indexof the non-linear material by an amount sufficient to induce switchingof the light from the first optical light source, whereby said switchingtakes place through the effect of leaking light from the first lightsource from the inside of the guiding core to the outside of the guidingcore. Here, the micro-structured optical waveguide may be or comprise amicro-structured optical waveguide selected from any of the embodimentsof the optical waveguide according to the second aspect of theinvention. However, the micro-structured optical waveguide mayalternatively be or comprise an optical planar waveguide.

According to a fifth aspect of the present invention there is providedan optical intensity limiting device comprising a micro-structuredoptical waveguide including sections comprising a material having anindex of refraction which changes as a non-linear function of lightintensity, said waveguide including a light guiding core region, andsaid waveguide being dimensioned for providing spatial overlap betweenthe sections filled with a non-linear material and light propagatingwithin the waveguide. The intensity limiting device further comprises afirst optical light source for introducing light into said waveguide ina mode guided along the core in such a manner that it illuminates thesaid sections filled with a non-linear material, and having a stronglyincreasing propagation loss for optical powers above a certain thresholdvalue. Here, the micro-structured optical waveguide may be or comprise amicro-structured optical waveguide selected from any of the embodimentsof the optical waveguide according to the second aspect of theinvention. However, the micro-structured optical waveguide mayalternatively be or comprise an optical planar waveguide.

For the devices or systems according to the present invention it iswithin preferred embodiments that the micro-structured waveguidecomprises waveguide features containing a photo-sensitive material inwhich permanent refractive-index changes may be induced. Thus, it may beadvantageous to combine the attractive features of the describedwavelength converter, with additional optical functionalities obtainedthrough the inclusion of waveguide features, where features of themicro-structure are containing a photo-sensitive material (e.g., for theability to write permanent refractive index structures in the waveguide,and hereby tune the properties of the waveguide converters).

It should be noted that the present invention is not limited to cases inwhich only two wavelengths are involved in the operation of the systemor device. It will also be a possibility to design micro-structurednon-linear optical waveguides for operation of more than two involvedwavelengths, e.g., for the development of new optical components, whichapplies four-wave mixing (FWM). Such components could be relevant forprocesses such as mid-span spectral inversion or other advanced opticaltechniques. Thus, the devices or systems of the present invention alsocovers embodiments in which more than two involved wavelengths areinteracting.

Other interesting aspects of the optically controlled micro-structuredwaveguide comprising non-linear sections is that they allow the controlof other optical properties than wavelength conversion, since controlledcoupling between different waveguide sections—these having differentdispersion properties—potentially leads to the development of opticallycontrolled dispersion manipulating devices. We may envision thepossibility of designing adjustable dispersion compensating devices,e.g., for fine tuning of single-channel dispersion. In more generalterms, the micro-structured optically controlled non-linear waveguidemay also be used for optically controlled delay elements, e.g., throughoptically controlled coupling between different waveguide sections.

The optically controlled waveguide properties of the devices accordingto the invention may also in preferred embodiments be used asdemultiplexing elements in optical time division multiplexing (OTDM)systems, since synchronization of a bit stream at a given wavelength andpulses from a channels-selection-control light source (emitting pulsesat a lower repetition rate than the first bit-stream) could be used forthe subtraction of individual time OTDM channels. For properconfigurations, e.g., applying counter-propagating optical signals, thedevice may operate also if both bit streams operate at the samewavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

The functionality and additional features of the invention will becomedearer upon consideration of the different embodiments now to bedescribed in detail in connection with the accompanying drawings. In thedrawings:

FIG. 1 shows an example of the transversal cross section of amicro-structured waveguide/fibre having elements filled with non-linearmaterial;

FIG. 2 shows an example of the transversal cross section of amicro-structured waveguide/fibre having elements filled with non-linearmaterial, and having a raised cladding index region for enhanced modeleakage control;

FIG. 3 shows an example of the transversal cross section of amicro-structured waveguide/fibre having elements filled with non-linearmaterial and at least two core regions;

FIG. 4 shows an example of the transversal cross section of ahoneycomb-structured photonic bandgap fibre having elements filled withnon-linear material near the central core region;

FIG. 5 illustrates schematically the operation of a so-calleddepressed-cladding fibre having an effective index modification andresulting signal mode control due to elements filled with non-linearmaterial;

FIG. 6 shows a band diagram for a honeycomb-structured photonic bandgapfibre illustrating the modification of the propagating properties of thecore mode as the control power is switched from low to highintensity—and vice versa;

FIG. 7 shows the mode field distributions for a honeycomb-structuredphotonic bandgap fibre design for low intensity control power levels aswell as for high intensity power levels;

FIG. 8 illustrates a schematic diagram of a micro-structured wavelengthconverting waveguide device in accordance with the invention, in whichthe wavelength to be encoded and the encoding wavelength areco-propagating;

FIG. 9 illustrates a schematic diagram of a micro-structured wavelengthconverting waveguide device in accordance with the invention, in whichthe wavelength to be encoded and the encoding wavelength arecounter-propagating;

FIG. 10 shows a schematic illustration of micro-structured wavelengthconverting waveguide device in which the encoding wavelength (or controlwavelength) is illuminating the micro-structured fibre from the side;

FIG. 11I shows an example of a micro-structured planar waveguidestructure in which the optical modes is guided in-plane orthogonal tothe elements filled with non-linear material, and allowing for opticalcontrol of the waveguiding properties and direction of the light;

It is to be understood that these drawings are for the purpose ofillustrating the concepts of the invention and they are, therefore, notmade to scale.

DETAILED DESCRIPTION OF THE INVENTION

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

This description is divided into four parts. Part I describes thefundamental nature and qualitative functionality of the micro-structurednon-linear waveguide. Part II describes the qualitative function of anoptically controlled switch according to the invention, Part IIIdescribes preferred wavelength conversion devices using the opticallycontrolled gating element based on an exemplary micro-structurednon-linear waveguide gate, and Part IV describes alternative approachesin the optical control of micro-structured non-linear waveguidingdevices.

I. The Optically Controlled Micro-Structured Waveguide

The waveguide can be either an optical fibre or a planar waveguide.

At low power levels, the optical modes propagate through the unperturbed(or only weakly perturbed) waveguide structure.

Referring to the drawings, FIG. 1 schematically illustrates thetransversal cross section of a micro-structured optical waveguide madeof a background material (11) having near core elements (12) filled witha material having an index of refraction, which is a non-linear functionof light intensity. Such non-linear materials could for example beorganic dyes having a non-linear absorption or refraction. The elements(12) could for some waveguide designs preferably be made with arefractive index that matches the refractive index of the backgroundmaterial (11) for small light intensities, and changes for higher lightintensities. Yet for other embodiments, it will be most advantageous tohave a non-linear index of refraction, which deviates from that of thebackground material at lower light intensities. The background materialwould in a preferred embodiment be made of glass, but otherpossibilities such as polymers are also included by the invention. Thewaveguide described in FIG. 1 has a core region (13), which in apreferred embodiment will have an index of refraction, which is higherthan the index of refraction of the background material. This may forexample be accomplished as in standard optical fibres by weakly dopingthe glass in the core region by materials such as germanium oraluminium.

When an electromagnetic field (preferably an optical field) with higherintensity is propagating through the micro-structured waveguide, theoptical properties of the non-linear material in the waveguide featuresare modified, and the waveguiding properties of the high intensity modefield as well as the waveguiding properties of other electromagneticfields are modified.

For the optical limiters based on the PBG effect as described by Shirkand Rosenberg, the optical fields were not guided through a waveguidingstructure. In contrast to these intensity dependent optical materials,the present inventors have realised how to increase the non-linearresponse significantly by the application of the waveguiding property.In contrast to prior art, we obtain the possibility of controlling lightbetween guided and non-guided (or leaky) modes, whereby much lower powerlevels are necessary for the non-linear property to be used. Thisenhanced effect will allow us to design waveguiding devices according tothe invention, which are compact and require relatively low opticalpowers to work.

In FIG. 2 another example of a waveguide according to the invention isillustrated. In this case the waveguide has a core (20) surrounded by ainner cladding background material (21) in which a microstructure withfeatures (22) containing optically non-linear material are located. Inthe present example the micro-structured features are placed in aclose-packed arrangement, but it will not be a requirement for all thewaveguides covered by the invention that the micro-structure necessarilyhas to show periodicity. The waveguide structure illustrated in FIG. 2further contains a cladding region (23) of a different material and/or adifferent refractive index than that of the inner cladding backgroundmaterial (21). The function of this cladding region (23) is that it willwork as a waveguide zone into which the mode, which is guided within(and/or in the near vicinity of) the core region of the waveguide at lowintensities, will leak into at higher intensities due to the non-linearproperty of the micro-structured features in the inner cladding region.It should be stressed that the circular shape of the cladding region(23) is not a requirement for the waveguide to work according to theinvention, and it could actually be a further advantage to operate witha non-circular cladding region (such as the ones often applied incladding pumped active optical fibres), because it may enhance thecladding mode dissipation and, prevent mode interference patterns todisturb the operation of the waveguiding device. Yet in other cases, theinteraction between core modes, and specific cladding modes may beenhanced through specific design of the magnitude and shape of the outercladding region (23), and in such cases a ring shaped form asillustrated in FIG. 2 may prove advantageous. In FIG. 2, we have finallyillustrated an outer fibre region (24), which may be index matched tothe leakage-control cladding region (23), or have a refractive index,which may enhance the mode leakage in some cases (higher index) or inother cases may improve the control of specific cladding modes(typically having a lower refractive index in region (24)).

One of the desired features of an optical converter is that it has ahigh dynamic range, because signal levels in the switch blocks will bepath dependent. A unique feature of the present invention is that itallows for a direct integration or inclusion of amplifying parts (erbiumdoping) within or in combination with the micro-structured waveguide.Thereby, the device will have the potential of adapting to differentsignal power levels, leading to a higher dynamic range.

FIG. 3 illustrates another example of a fibre cross-section according tothe invention. In this case, the fibre has at least two core regions(31) and (32). These core regions may in a preferred embodiment beseparated so much that only very limited power is coupled between themfor transmission of low power levels. However, as the optical powerlevel (from either the signal itself, or from a control light source) isincreased, the refractive index of the micro-structured features (33)containing optically non-linear material will change, leading to aleakage of optical power between the core regions (31) and (32). Thefibre background material (34) may be realised in materials such asglass, polymers or other suitable materials. In another preferredembodiment, the coupled waveguide core regions may have a significantcoupling—even in the case of low intensity power propagation—and in thiscase the non-linear micro-structured features (33) will act as means ofmodifying the coupling length of the waveguide device. It should also benoted that the core regions may be realised through standard waveguidetechnology using total internal reflection as the guiding principle, or(for periodic structures) as photonic bandgap guiding waveguides. Inboth cases unique waveguiding properties may be obtained, and the devicewill allow optical signal power to be switched between waveguide coreregions.

In FIG. 4, another preferred embodiment of a wavelength convertingwaveguide device is schematically illustrated. FIG. 4 shows thecross-section of a honey-comb structured photonic bandgap fibre,consisting of a core region formed by a defect (40)—which preferablycould be a void—located within a periodically distributedmicro-structure of features (42), providing the photonic bandgap. Thecladding structure features (42) may also typically be voids, and theyare in this example all placed in a homogeneous background materiallocated within a section (43) of the fibre, large enough to provide alocal confinement of the mode field, when low-intensity optical signalsare transmitted. The structure also includes micro-structured features(41) containing optically non-linear material, and the fibre couldoutside the PBG region be over-cladded by a homogeneous material (44).In the described example of FIG. 4, the fundamental operation of thedevice is that the core defect (which has an average index below that ofthe fundamental space filling mode of the cladding) is modified by anincrease of the refractive index of the features containing non-linearmaterial. Hereby a threshold is reached, at which the signal no longeris guided, and for powers above this threshold, the signal becomeshighly leaky. Now, if the non-linear refractive index of the features(41) is controlled by an optical mode (the control light) launched atanother wavelength than the first mentioned signal source, and thecontrol light is modulated, then it will be possible to transfer thismodulation to the first signal wavelength. We will in an additionalexample in the following text describe, how the combination ofnon-linear micro-structured features and TIR-guided modes in contrast toPBG-guided modes will indicate preferred relations between thewavelengths of the encoding signal, and the encoded signal.

II. Optical Switch

FIG. 5 provides a schematic illustration of the physical principlebehind a preferred embodiment of the optically controlledwaveguide-switching element according to the invention. In a preferredembodiment, such as the one illustrated here, the fundamentalwaveguiding principle is total internal reflection, and theseTIR-guiding waveguides are considered realised as depressed claddingstructures in which the refractive index of the core (50) issubstantially equal to the refractive index of the outer cladding (51).If the inner cladding region contains micro-structured features (52)filled with an optically non-linear material, and we, furthermore,assume the refractive index of these features to be index-matched withthe background material of the inner cladding region at low lightintensities, then the optical mode may be confined to the core region ofthe fibre. This is further illustrated on the lower part of the figure,where the index profile (54) of the waveguide is shown for two differentwavelengths λ₁ (dashed curve (55)) and λ₂ (solid curve (56)). In thepresent example, the wavelengths are chosen so that λ₁<λ₂. As theintensity of the light is increased, and we here assume for simplicitythat it is the intensity of the control light at wavelength λ₁, which isincreased, the result of this intensity adjustment is that the opticallynon-linear material in the features (53) increases its refractive index.In the lower illustration on the right-hand side of FIG. 5, this isillustrated as a lifting of the average refractive index in thedepressed cladding section of the index profile (57). As a consequenceof this modified refractive index profile, the optical modes of thewaveguiding structure are transversally changed, and as a result thecontrol mode at wavelength λ₁ is somewhat broadened, however, thetransversal modification is much stronger for the wavelength λ₂, whichbecomes highly leaky. This effect may be enhanced further, e.g., bybending the optical fibre, but the consequence will in any case be asignificant change in optical transmission loss for the wavelength λ₂.It is hereby described, how we may obtain an optically controlledswitch, in which the attenuation of the signal at wavelength λ₂ may bealtered by controlling the intensity of the optical power carried by thecontrol wavelength λ₁. It should in connection with the discussion ofFIG. 5 be noted that the mode fields are shown on an arbitrary amplitudescale, and the illustrations are, therefore, not indicating the powerlevels necessary for the switching procedure. However, we stress thatthe optical power levels due to waveguiding should be significantlylower than those obtained by Shirk and Rosenberg, where optical limitingwas reported at 250 nJ.

After now having discussed the switching ability of the high-index corefibres operating by the TIR-like principle, we will now turn to apreferred embodiment applying a PBG waveguide. In the chosen example, ahoneycomb-structured photonic bandgap fibre with a low-index core regionhas been chosen, and in FIG. 6 the band diagram for the honeycomb fibredesign is shown. We here refer to a waveguiding structure as the oneshown in FIG. 4, and the example is chosen with a honeycomb-structuredcladding with distance Λ between the air-filled holes, which are placedin a silica background material. In this particular example, the holediameter of these cladding features are given as d=0.40·Λ, and forlow-intensity optical powers propagating through the structure, thefeatures (41) containing optically non-linear material are assumed tohave the same refractive index as the silica background material. Theguided mode field is, therefore, confined to the core region by theair-filled core hole (often denoted “the defect” of the photonic bandgap waveguide), which in the specific example chosen here has a diameterof d_(core)=0.40·Λ. In FIG. 6 is shown the photonic band gaps, which aretypical for the honeycomb-structured cladding of the example, andlocated within this bandgap we find the solution for the guided moderelated to the core defect of the fibre waveguide. We note that theguided mode solution enters the band gap from the top (around anormalised frequency of Λ/λ=0.6) and traverses it before it exits at thelower band gap boundary for a normalised frequency around Λ/λ=2.0. Itshould be pointed out that the physical effect of the mode movingoutside of the band gaps is that the guided mode will not immediatelycease to exist, but it will be co-propagating with the continuum ofleaky cladding modes, and it will in practice couple to these and herebythe power will be rapidly dissipated. We may, therefore, in practiceconsider the core mode to leak its power over very short distances, whenthe solution exits the band gap.

We will now use this effect to obtain the desired switchingfunctionality described by this invention, and, furthermore, obtain somepreferred relations between the wavelengths of signal light and controllight, respectively. We will, therefore, start by directing this exampletowards the most feasible waveguide designs, namely those having thelargest possible dimensions (the hole distance Λ). In practice, thismeans that we will consider placing both signal wavelength and controlwavelength in the high-value end of the guided mode band illustrated inFIG. 6 (i.e., relatively close to the point, where the core modesolution traverses below the lower band gap limit at Λ/λ=2.0). Now asthe power level is increased for the control light, the non-linearmaterial in the features located in the near core region of the fibrewaveguide will get a higher refractive index (than that of the silicabackground material). Since the location of the optical band gap isdetermined by the cladding structure, the limits of the optical band gapwill not be influenced. However, the modification of the refractiveindex will result in a movement of the guided mode solution of the coremode, and as illustrated in FIG. 6 the guided mode curve for thehigher-intensity power solution is moved upwards compared to thelow-intensity solution. If the signal wavelength is placed such that anin-band solution is obtained for low-intensity propagation, but outsidethe band for high-intensity control light, the signal will move from astate of being strongly confined to the core region in the low-intensitycase to a strong leakage in the high-intensity case. A possible locationof such a signal mode for the specific example will be for Λ/λ₂=2.2,where the free space wave number k₂=2π/λ₂, λ₂ being the signalwavelength. If the control mode—at wavelength λ₁—has to be able tomaintain the high intensity (and thereby modify the optical waveguide asdesired), it is for the specific example essential that the control modestay guided. This means that the control mode should be chosen for afree space wave number k₁, which is so much smaller than that of thesignal mode that the mode solution of the control mode will stay withinthe band gap (in this example a value of Λ/λ₁=1.6 could be apossibility). In this example, we have, therefore, found that the signalwavelength (λ₂) and control wavelength (λ₁) preferable obey the relationλ₁>λ₂.

For the fibre of this specific example, a signal wavelength, λ₂, of 1.55μm would demand a centre-to-centre spacing of two nearest air holes inthe cladding of about 3.5 μm, and a control wavelength, λ₁, of about 2.0μm. However, a shorter control wavelength (closer to the signalwavelength) may also be obtained for other designs than the onediscussed above. For example could the amount of non-linear material beincreased or decreased in order to change the high-intensity cut-off ofthe signal wavelength—thereby allowing to push or expel the controlsignal wavelength against or away from the signal wavelength. It is,however, often desired to have the signal and control wavelength closelyspaced, hence to have a spacing of less than 500 nm such as less than300 nm (for example at 1.3 μm and at 1.5 μm), or even closer as forwavelength division multiplexing systems—where a spacing as small as 10nm is desired. For other applications, it is required that the signalwavelength is shorter than 1.55 μm, e.g., such as around 800 nm, hencepreferred embodiments cover fibres with a centre-to-centre spacing ofnearest air holes smaller than 3.5 μm, such as around 3.0 μm, or such asaround 2.0 μm, or even smaller.

It should be noted that in the described example, the upper end of theguided mode solutions with regard to dimensions of the PBG structure waschosen, However, as the technology is further developed, a developmenttowards high precision control of even very small structureperiodicities are expected, and in this case it will also becomefeasible to use the other end of the photonic band gap, and herebyinverting the relation between wavelengths of signal and control light.

FIG. 6 furthermore illustrates this relation for the above-discussedfibre design, when the fibre is operated in the low-frequency range. Thefigure allows to correctly design the dimensions of the fibre. If, forexample, the signal wavelength is desired to be at 1.55 μm, then thecentre-to-centre spacing of two-nearest air holes should be about 1.0μm—and the control wavelength should be around 1.3 μm. For otherapplications, it is required that the signal wavelength is shorter—suchas around 800 nm—hence, the centre-to-centre spacing of nearest airholes should be even smaller than 1.0 μm.

Further optimisations of the fibres according to the present inventionmay be obtained by varying the fraction of air holes and/or non-linearmaterial. For certain applications, a large separation of the twowavelengths is desired—whereas for other applications, a smallseparation is desired. Independent tuning of the filling fractions ofholes and non-linear material provides a large flexibility for designingfibres for specific applications.

Yet another aspect of the invention is that in the case of both signaland control modes being located such that both falls outside the bandgap of the modified waveguide (the high-intensity case), we will see adifferent operation of the device. In this case, the control modeshifting from low-intensity to high-intensity will lead to the leakageof the signal mode, but also to the leakage of the control mode itself.This can result in much lower local mode field intensity for the leakymode, and, consequently, the waveguide will switch back to thelow-intensity case. As this happens, the control mode intensity willagain reach its guided form and the threshold for modifying thenon-linear material will once again be reached, where after the wholeprocedure repeats itself. This means in other words that we haveobtained an optical oscillator operating by a frequency given by theresponse time of the non-linear material and (to a minor degree) theguided mode properties and the waveguide properties of the photonic bandgap fibre.

To illustrate the principle of the honeycomb-structured PBG waveguidefurther, we have in FIG. 7 shown the calculated mode fields for astructure with the air hole diameter, d, given in relation to the holespacing, Λ, as d/Λ=0.4, i.e., the air-filling fraction for thehoney-comb cladding design is 10%.

III. All-Optical Wavelength Converter

After having described, how the principle of optically controlledwaveguiding in a micro-structured fibre/waveguide may be realisedaccording to the present invention, we will address the use of suchwaveguides as wavelength conversion elements by drawing forward someexamples of how to place them in proper optical set-ups.

In FIG. 8, a schematic diagram of a micro-structured wavelengthconverting waveguide device in accordance with the invention is shown.Here, the encoding wavelength (the wavelength, λ₁, which carries theencoded information) is provided from a device (80) as shown at FIG. 8.It should be noted that this encoding device could be any kind ofoptical light source (e.g., a laser or light emitting diode) incombination with a proper modulating device. The element (80), asindicated on the illustration may also represent an optical transmissionline—as a part of an optical communication system—and in this case itmay even include optical amplifiers, dispersion manipulating devicesetc. to provide the desired signal quality. FIG. 8 further shows anotherlight source (81) operating at wavelength λ₂. This light source may forsimplicity be considered as a continuous-wave (CW) source, to which theencoded information has to be converted by the wavelength convertingdevice. Also in this case may the element (81) include amplifiers. Thelight from the two light sources (80) and (81) are guided by opticalwaveguides (e.g., optical fibres) to a combining device (82) (forexample a dicroic coupler) from which the light is guided to thewavelength converting device (83), including a micro-structured sectionwith features filled with a non-linear material according to theinvention. We note that in the example presented in FIG. 8, thewavelength to be encoded, λ₂, and the encoding wavelength, λ₁, areco-propagating. For certain applications it will, therefore, benecessary to include an optical filtering (85) after the waveguide (84),through which the optically processed signal is guided after wavelengthconversion. Such cases may for instance be situations where a part ofthe encoding wavelength is coupled out through the waveguide (84), andwhere it further would be a disadvantage for the optical system, thatthis wavelength entered the following transmission link or opticalnetwork. After the filtering device (85), the signal encoded onwavelength λ₂ is now transmitted through the waveguide (86), which alsomay include a transmission link, a part of an optical network etc.

Yet another example of a wavelength-converting device is outlined inFIG. 9, which illustrates a counter-propagating configuration forencoding and encoded wavelengths, respectively. The example shown inFIG. 9 includes an optical light source (91) (e.g., a laser or a lightemitting diode)—including the necessary amplifying and filteringcomponents—from which a CW optical field centred around the wavelengthλ₂ is emitted through a waveguide (92), which for example may be anoptical fibre, a planar optical device, or a system of bulk opticalcomponents. The waveguide (92) is connected to one end of themicro-structured optical waveguide (94) that includes features filledwith non-linear material, and operating as indicated by the previousexamples. In contrast to the previously described converter design, theencoding wavelength, λ₁, is here counter-propagating with respect to thefirst mentioned wavelength λ₂. The encoding wavelength is emitted fromthe light source (96) and guided into the non-linear optical device (94)through a coupler (95) (e.g., a dicroic coupler) and the waveguide (93).It should be noted that the waveguide (93), which here operates as theout-coupling waveguide for wavelength λ₂ also may be a combination ofbulk optical components, but preferably will be a fibre or a planarwaveguide component. After the encoding on wavelength λ₂ has takenplace, the wavelength-converted signal is now guided through theout-coupling section (93) through the coupler (95) and onto thetransmission line (97) or other part of the optical system in which thewavelength converter is an element. The counter-propagating set-up asdescribed in FIG. 9, has the advantage of allowing a better wavelengthseparation between λ₁ and λ₂.

It should be noted that the configurations described in FIG. 8 and FIG.9 also have the possibility of operating as optically controlledrouters. In these cases, the control wavelength λ₁, would either beswitched on or off, and depending on the optical intensity of thiscontrol wavelength, the wavelength λ₂, which in such cases then wouldcarry the signal information, would be either guided through thenon-linear component or attenuated. Such a device will be even moreefficient, if the two-core device—as indicated in FIG. 3—is combinedwith a coupling device that allows the output bit stream to be switchedbetween waveguide paths.

IV. Alternative Realisations of Optically Controlled Non-LinearWaveguiding Devices.

In the previous sections, we have described a number of examplesaccording to the present invention of optically controlled non-linearwaveguiding devices and their applications within switching and opticalwavelength conversion. In these examples it has been natural to assumethat all optical fields have been propagating predominantly in the samedirection or in opposite directions. However, as the opticallycontrolled waveguide according to this invention provides a highlysensitive functionality (i.e., low powers and short waveguide lengthsare needed to obtain the described functionality), it will be an obviouspossibility to provide separate propagating directions for differentwavelengths. This will clearly lead to an increased flexibility in thedesign of future optical components according to the describedinvention.

A first example of such a separation of optical waveguiding directionsbetween different wavelengths is presented in FIG. 10, where amicro-structured optical fibre containing features filled withnon-linear material is illuminated from the side. The side illuminationby the control light (101) is provided through a focusing device (102)onto the core region (103). By controlling the intensity of the light(101), the light guiding property of the fibre waveguide core region maybe changed, and the previously described properties obtained.

A second example of a waveguiding device according to the invention, inwhich the propagating directions of the involved wavelengths areseparated, is shown in FIG. 11. This figure illustrates a top-down viewon a planar optical waveguide structure operating by the photonicbandgap principle. As it is well known to those skilled in the art,planar photonic bandgap devices holds the potential of obtaining muchsmaller integrated optical waveguide component sizes than possibly usingpresent days technology. One of the reasons for this is that very sharpwaveguide bends may be realised in planar PBG waveguides, and only a fewlayers of periodically distributed waveguide features are necessary forobtaining the waveguide effect. It is, therefore, an interestingpossibility to be able to optically switch the power in such waveguidedevices. In the schematic illustration in FIG. 11, the signal ispropagating (in the plane of the figure) with a predominant directionindicated by the arrow (111). For low intensities, and for lowintensities of the control light (112), the signal will be guided aroundthe sharp bend and exit in the direction indicated by the arrow (113).However, if the periodically placed features (115) contains opticallynon-linear material as previously described, then it will be possible tomodify the refractive index of these features and thereby change thebandgap properties given by the periodically placed features and thebackground material (116), in which they are placed. If we—in analogy tothe example described in FIG. 6—move from a situation, where the signalmode is guided to a leaky mode situation, the output at (113) will bestrongly affected by the intensity of the control light entering throughthe waveguide (114). Note that the waveguide for the control light maybe a classical TIR based waveguide or even top illumination of theplanar waveguide using micro-lens systems of fibre coupling. It shouldalso be noted that the type of planar waveguide devices in which thiseffect may be used are much broader than indicated by this example,since coupling between a multitude of waveguide cores, or opticalcontrol of resonators are among the unique possibilities of theprinciple described in this invention.

The invention being thus described, it will be apparent that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be recognized by one skilled in the art areintended to be included within the scope of the following claims.

1. An optical wavelength conversion device comprising: amicro-structured optical waveguide including sections that have anon-linear material with an index of refraction which changes as anon-linear function of light intensity, said waveguide including a lightguiding core region, and said waveguide being dimensioned for providingspatial overlap between the sections filled with the non-linear materialand light propagating within the waveguide; a first optical light sourcefor introducing light into said waveguide in a mode guided along thecore; a second intensity modulated light source for introducing encodinglight into said waveguide in such a manner that it illuminates thesections filled with a non-linear material, said second light sourcehaving an intensity modulation sufficient to change the refractive indexof the non-linear material by an amount sufficient to encode or modulatethe light from the first optical light source accordingly, said encodingtaking place through the effect of leaking light from the first lightsource from the inside of the guiding core to the outside of the guidingcore.
 2. A micro-structured optical waveguide having an axial directionand a cross section substantially perpendicular to said axial direction,the micro-structured optical waveguide comprising sections that areelongated in said axial direction and which include a non-linearmaterial having an index of refraction which changes as a non-linearfunction of light intensity, said waveguide including a light guidingcore region that is elongated in the waveguide axial direction and has adiameter larger than 2 microns, and said waveguide being dimensioned forproviding spatial overlap between the sections filled with thenon-linear material and the light propagating within the waveguide, saidwaveguide further including a cladding zone to which the light may beleaking at high intensities, an inner boundary of said cladding zonebeing placed at a distance from the core centre which is larger than0.75 times the core diameter.
 3. A micro-structured optical waveguidehaving an axial direction and a cross section substantiallyperpendicular to said axial direction, the micro-structured opticalwaveguide comprising sections that are elongated in said axial directionand which include a non-linear material having an index of refractionwhich changes as a non-linear function of light intensity, saidwaveguide including a light guiding core region that is elongated in thewaveguide axial direction and surrounded by a cladding region, and saidwaveguide being dimensioned for providing spatial overlap between thesections filled with the non-linear material and the light propagatingwithin the waveguide, the waveguide core region being realised with alowered refractive index compared to the refractive index surroundingthe core, and said waveguide including a multiplicity of spaced apartcladding-structure features that are elongated in the waveguide axialdirection and disposed in the cladding region surrounding said coreregion, the waveguide cladding structure features including air holeswith a centre-to-centre spacing of nearest air holes being smaller than5 μm.
 4. A micro-structured optical waveguide having an axial directionand a cross section substantially perpendicular to said axial direction,the micro-structured optical waveguide comprising sections that areelongated in said axial direction and which include a non-linearmaterial having an index of refraction which changes as a non-linearfunction of light intensity, said waveguide including a light guidingcore region that is elongated in the waveguide axial direction andsurrounded by a cladding region, and said waveguide being dimensionedfor providing spatial overlap between the sections filled with thenon-linear material and the light propagating within the waveguide, thewaveguide core region being realised with a lowered refractive indexcompared to the refractive index surrounding the core, and saidwaveguide including a multiplicity of spaced apart cladding-structurefeatures that are elongated in the waveguide axial direction anddisposed in the cladding region surrounding said core region, thewaveguide sections that include the non-linear material having across-section area which is at least 10% as large as a cross-sectionarea of the cladding-structure features.
 5. A micro-structured opticalwaveguide having an axial direction and a cross section substantiallyperpendicular to said axial direction, the micro-structured opticalwaveguide comprising sections that are elongated in said axial directionand which include a non-linear material having an index of refractionwhich changes as a non-linear function of light intensity, saidwaveguide including a light guiding core region that is elongated in thewaveguide axial direction, and said waveguide being dimensioned forproviding spatial overlap between the sections filled with thenon-linear material and the light propagating within the waveguide, saiddimensioning of said micro-structured optical waveguide being such thatthe sections that include the non-linear material are placed within adistance from the core centre which is smaller than 10 times theoperating wavelength of the light to be guided by said core region. 6.The micro-structured optical waveguide according to claim 5, wherein thecore region is surrounded by a cladding region.
 7. The micro-structuredoptical waveguide according to claim 6, wherein the waveguide coreregion is realised with a lowered refractive index compared to therefractive index surrounding the core, and in which the waveguideincludes a multiplicity of spaced apart cladding-structure features thatare elongated in the waveguide axial direction and disposed in thecladding region surrounding said core region.
 8. A micro-structuredoptical waveguide according to claim 6, wherein the sections includingthe non-linear material are voids or channels elongated in the axialdirection of the waveguide and arranged in the cladding regionsurrounding the core region.
 9. The micro-structured optical waveguideaccording to claim 6, wherein the sections including the non-linearmaterial are periodically arranged in the cladding region surroundingthe core region.
 10. The micro-structured optical waveguide according toclaim 5, wherein the waveguide core region is realised with a raisedrefractive index compared to the refractive index surrounding the core.11. The micro-structured optical waveguide according to claim 5, whereinthe micro-structured optical waveguide is an optical fibre.
 12. A Themicro-structured optical waveguide according to claim 5, wherein thecore region has a diameter larger than 2 microns.
 13. A micro-structuredoptical waveguide having an axial direction and a cross sectionsubstantially perpendicular to said axial direction, themicro-structured optical waveguide comprising sections that areelongated in said axial direction and which include a non-linearmaterial having an index of refraction which changes as a non-linearfunction of light intensity, said waveguide including a light guidingcore region that is elongated in the waveguide axial direction andwaveguide features that contain a photo-sensitive material in whichpermanent refractive-index changes may be induced, said waveguide beingdimensioned for providing spatial overlap between the sections filledwith the non-linear material and the light propagating within thewaveguide.
 14. A micro-structured optical waveguide having an axialdirection and a cross section substantially perpendicular to said axialdirection, the micro-structured optical waveguide comprising sectionsthat are elongated in said axial direction and which include anon-linear material having an index of refraction which changes as anon-linear function of light intensity, said waveguide including a lightguiding core region that is elongated in the waveguide axial directionand surrounded by a cladding region, and said waveguide beingdimensioned for providing spatial overlap between the sections filledwith the non-linear material and the light propagating within thewaveguide, said waveguide further including a multiplicity of spacedapart cladding-structure features that are elongated in the waveguideaxial direction and disposed in the cladding region, the saidcladding-structure features predominantly being periodicalcladding-structure features.
 15. The micro-structured optical waveguideaccording to claim 14, wherein the cladding-structure features areplaced in a close-packed arrangement.
 16. The micro-structured opticalwaveguide according to claim 14, wherein the waveguide claddingstructure features are air holes.
 17. An optical switching devicecomprising: a micro-structured optical waveguide having an axialdirection and a cross section substantially perpendicular to said axialdirection, the micro-structured optical waveguide including sectionsthat have a material with an index of refraction which changes as anon-linear function of light intensity, said waveguide including a lightguiding core region, and said waveguide being dimensioned for providingspatial overlap between the sections filled with a non-linear materialand light propagating within the waveguide; a first optical light sourcefor introducing light into said waveguide in a mode guided along thecore; a second variable intensity light source for introducing lightinto said waveguide in such a manner that it illuminates the sectionsfilled with a non-linear material, said second light source having anintensity variation sufficient to change the refractive index of thenon-linear material by an amount sufficient to induce switching of thelight from the first optical light source, said switching taking placethrough the effect of leaking light from the first light source from theinside of the guiding core to the outside of the guiding core.